This invntion relates to a process for producing a composite consisting essentially of a titanium diboride reinforcement phase dispersed in a metal matrix which can be aluminum metal or an aluminum based alloy.
Aluminum based alloys are relatively light weight and low cost materials and thus desirable for use in aerospace applications. Their use however, have been limited to low temperature applications because of rapid degradation of their mechanical properties at temperatures above about 250.degree. F. Development of aluminum alloys with adequate mechanical properties at higher temperatures would be highly desirable since these could be used to replace more expensive titanium based alloys.
At present, development of these high temperature aluminum alloys is based on two key concepts or technologies, that is, (1) rapidly solidified alloys and (2) metal-matrix composites. The rapid solidification method is based on the principle that rapid cooling during the solidification process results in refined microstructures and/or super saturation of the metal matrix with alloying elements resulting in increased precipitation hardening upon using suitable heat treatment. Atomization and melt spinning are two of the techniques used to achieve the high cooling rates. Alloying elements used to impart the desirable high temperature properties have low solubility and diffusivities in the metal matrix and precipitate out as intermetallic compounds. Alloys being developed are based on the systems. Al-Fe-Ce Al-Fe-Mo, Al-Ti-Hf, Al-Cr-Zr, etc. The high temperature emchanical properties of these rapidly solidified alloys is dependent on the thermal stability of the precipitated phases. Though the improvements in the high temperature mechanical properties of these advanced powder metallurgy aluminum base alloys have been impressive, they lack specific strength equivalency with titanium based alloys.
The second methof of producing a high temperature high strength aluminum based system is based on the composite approach. For example, discontinuously reinforced aluminum alloys fabricated via powder metallurgy processing represent a maturing technology offering aluminum based alloys having improved specific stiffness and strength at only a slight increase in density. Silicon carbide whisker or particulate-reinforced aluminum alloys are fabricated using the composite approach. The process for fabricating whisker reinforced materials on a commercial basis has been developed by ARCO Metal's Silag Operation. U.S. Pat. No. 4,259,112 assigned to DWA Composite Specialities, Inc. describes a method in which a binder is utilized to make green "pancakes" of SiC and aluminum powders which are then stacked prior to hot pressing. While the preliminary steps to fabricate a billet are somewhat different, both ARCO and DWA processes involve vacuum hot pressing slightly above the solidus temperature to achieve full densification of billet and plate. Subsequent extrusion or forging of the billet is necessary to optimize mechanical properties.
The apparent need to hot press at a temperature above the solidus temperature of the alloy, (that is, the alloy is partially remelted) to achieve wetting of the silicon carbide reinforcement is a limitation of the process since the solidification rate experienced by the remelted matrix is comparatively much slower than that of the starting powder material used as the metal matrix. Thus, the melting and resolidification cycle used in the process destroys the desirable rapidly solidified structure of the starting powder. The resulting alloy segregation can be deleterious in terms of the mechanical properties of the matrix and hence of the composite system.
Another composite technique called "compocasting" invovles adding non-metals to partially solidified alloys. The high viscosity of the metal slurry prevents particles from settling, floating, or agglomerating. Bonding of non-metal to metal is accomplished by interaction between the respective particles. Mehrabian, R., Riek, R.G., and Flemings, M.C., "Preparation and Casting of Metal-Particulate Non-Metal Composities," Metall. Trans., 5(1974) 1899-1905 and Mehrabian, R., Sato, A., and Flemings, M.C., "Cast Composites of Aluminum Alloys," Light Metals, 2(1975) 177-193. The cooling rates experienced by the metal-matrix are again low, comparable to other casting techniques (10.sup.-3 to 1.degree. C./sec.). Still another method for producing powder metallurgy composite materials is by mechanical alloying. This is essentially a high energy ball milling operation which is done typically in a stirred ball mill called an attritor mill. High strength material results from mechanically working the alloy because of incorporation of oxides and carbides during the milling operation, and because of strengthening mechanisms due to severe working resulting in fine grain and sub fine grain size.